1. Field of the Invention
The present invention relates to a control system for positioning of a data transducer on data tracks in a disk storage drive system and more specifically, a data transducer position control system for disk storage drive system involving a method and system of moving a data transducer at a high speed to the selected data track and also, controlling the relative position of the data transducer to the data track.
2. Description of the Prior Art
As there has been improved an information recording and reproducing apparatus, a position control system which is much smaller in size, consumes less power, and operates at a higher speed and with more accuracy, has been required for use as a positioning driving system for either a recording or reproducing transducer in a magnetic disk storage drive system or an optical disk storage drive system. Particularly, it is desired: speeding-up for achieving the high speed access corresponding to high speed operation in semiconductors; miniaturization for having a greater amount of capacity in a given size; low power consumption for allowing the system to become compact in size and portable; and high accuracy for increasing the density of tracks and thus, the capacity of memory.
Such a position control system commonly employs a stepping motor as a positioning device. For example, a Hybrid Permanent Magnet type stepping motor is most preferable in which a rotor comprises an axially magnetized permanent magnet and a couple of magnetic materials mounted on both sides of the permanent magnet and having arrays of magnetic teeth formed at equal pitches in the circumferential peripheries of the magnetic materials. Also, a stator in the motor comprises a magnetic material core having groups of magnetic teeth provided therein opposite to the rotor and a plurality of windings mounted on and around the magnetic material core.
The Hydrid Permanent Magnet type stepping motor is energized through a basic power amplifier circuit which includes bridge circuits having a plurality of switches and the windings located between the bridge circuits. When the windings are magnetized one after the other in order by activating the switches at regular intervals of time, the rotor advances from position to position continuously due to the series of magnetizing actions on the windings. The switches are commonly semiconductors.
More specificaly, a 2-phase feeding stepping motor is widely provided which allows its rotor to rotate (advance) 1/4 of the tooth-pitch (an electrical angle of 90 degrees) as the switches operate for opening and closing consecutively on synchronizing with exterior signals.
There has been introduced a prior art position control system of a disk system, as disclosed in U.S. Pat. No. 4,568,988, employing such a stepping motor and called as an open loop positioning system in which the bridge circuits are activated one after the other by the input of step pulses and thus, the rotor in the stepping motor advances forward in rotation to move a data transducer for accesss to tracks.
This prior art position control system includes the stepping motor as an electromagnetic positioning device in the open loop positioning system, thus allowing its electronic control circuit to be relatively simple in arrangement. Particularly, as the positioning is made by pulse feeding, a simple method is used in which pulses are directly supplied from a microprocessor to a power amplifier. Additionally, this system offers the higher effectiveness of power as compared to the other positioning device such as a voice coil motor or a servo motor (i.e. an actuator such as a stepping motor having tooth-shaped magnetic teeth in the magnetic circuit can provide high effectiveness of power and generate a great force or torque at a low rate of current), thus being compact in size and less in power consumption.
On the other hand, the system is yet disadvantageous in respect to high-speed operation and accuracy. Particularly, the stepping motor as a positioning device causes a data transducer to vibrate about its standstill point during positioning to the selected data track and thus, it may require a considerable setting time for positioning.
To reduce the vibration and minimize the setting time, the rotor and moving members need to be involved with mechanical viscous damping. This results in intricacy in the arrangement and requires a longer seek time for positioning due to the viscous damping. Such problems may not arise in the position control system incorporated with a voice coil motor or a DC servo motor. As the stepping motor is rotatable at a speed synchronized with outside pulse signals, the data transducer can perform high speed seek when the pulse signals are increased in frequency. However, during high speed operation, unlike low speed operation in which a propelling force is great, the rise-up of current is delayed due to a time constant of windings and hysterisis loss and the production of torque will be reduced in efficiency. Consequently, the stepping motor becomes liable to be out of stepping when the high speed seek is attempted. If the magnetic pole pitch is reduced in width in order to increase the resolution of positioning, the switching frequency increases and the disadvantage will be accelerated.
If the density of tracks is increased by reducing the distance between the two adjacent data tracks for the purpose of increasing the information recording capacity, the positioning needs to have higher accuracy. More particularly, the higher position holding ability (stiffness) against an outside vibration or impact is required and also, the results from track eccentricity or meander will be relatively unnegligible. Particularly in the prior art position control system, the magnetic pole teeth of the rotor and stator in the stepping motor can be increased in number to reduce the degree of step angle for precision in positioning and improvement of the resolution. Howver, in respect to the limitation of mechanical accuracy, the magnetic pole teeth cannot be increased in number. Even if the number is increased and the resolution of positioning is improved, it will still be difficult to obtain the absolute precision of positioning and increase the stiffness (holding torque). Additionally, the prior art position control system fails to provide satisfactory trackability due to the track eccentricity and meander inherent in the replaceable media
There has been known another method in which the high speed seek, higher track density and high precision trackability is purposed to improve a data disk storage drive system in performance by using the combination of a closed loop servo control and a position control system having a position device such as a voice coil motor or a DC servo motor. This type of system is however, as compared to the previously described system having the stepping motor, low in power effectiveness, great in power dissipation, unsuitable for miniaturization, and intricate in the arrangement of its electronic control circuit. More specifically, the circuit includes two loops; a position control loop for tracking control and a velocity control loop for seek control. The latter is provided for speed control with a table of velocity profile which is a function of the number of remaining tracks and should be replaced with the former position control loop in electrical control before the data transducer reaches a target track. Also, a velocity measuring circuit is substantially intricate in the arrangement.
Furthermore, according to the prior art data disk storage drive system having the stepping motor actuated by such a known open loop positioning control, the off-tracking of the data transducer cannot be detected at once when resulting from outside vibration or impact. It is considerably important to detect the off-tracking for the reason that the data writing should stop when the data transducer is displaced off the data track due to outside vibration or impact. In the prior art, the disk is provided with servo information embedded in the surface thereof for picking up the relative positioning error of the data transducer to the data track. The servo information is thus utilized for improved trackability and off-track detection.
A latest disk for use in the magnetic data disk storage drive system is provided with no such dedicated servo surface thereof for the purpose of increasing the memory capacity. Instead, the entire disk surface becomes a data recording surface in which pieces of the servo information are scattered. Even on an optical disk of the type with no special track groove, the servo information for tracking is provided in scattering arrangement. This is termed as a sampling servo method or a sector servo method. The sector servo represents the arrangement of servo information in every data sector, particularly as disclosed in U.S. Pat. No. 3,593,333. In such a method, information is given only intermittently due to the servo mechanism and thus, the improvement in trackability will be not easy to accomplish. Also, no off-tracking can be detected instantly upon occuring.
As described above, the prior art position control system for disk storage drive system, employed as a positioning device, using the stepping motor provides the advantage of compactness in arrangement or energy-saving and simultaneously, is disadvantageous in which the track positioning takes a considerable settling time and also, the high-speed seek is effected with difficulty. Additionally, in the respects of a construction of the stepping motor and an arrangement of the open loop positioning control, it is difficult to greatly improve the positioning resolution and increase the accuracy and stiffness in positioning and also, to ensure the trackability throughout the operation. On the other hand, the other known position control system of closed loop servo operation, using another positioning device such as a voice coil motor or a DC servo motor, involves the considerable loss of power and although may allow a data disk storage drive system to be easily improved in performance, is appropriate to no compactification and intricate in the arrangement of its electronic control circuit. As the track pitch in a latest data disk storage drive system is comparatively short, deflection of the data track on a replaceable media during rotation of the disk has to be much considered while affecting in trackability. Also, the off-tracking is possibly effected due to outside vibration and impact.